Method for producing eutectics as thin films using a quartz lamp as a heat source in a line heater

ABSTRACT

A method for the preparation of aligned eutectics as thin films is provided. The components of the eutectic are deposited in overlying planar layers on a suitable substrate to form a preform and a molten zone, having predetermined characteristics, is established and caused to traverse the preform melting and intermixing the deposited layers leaving the solidified thin film eutectic in its path.

CROSS-REFERENCE TO RELATED APPLICATION

This application is division of application Ser. No. 245,764 filed Mar.20, 1981.

The invention herein is related to the invention disclosed and claimedin Ser. No. 253,985, filed Apr. 13, 1981 now U.S. Pat. No. 4,349,621issued on Sept. 14, 1982, assigned to the same assignee as the instantapplication, and herein incorporated by reference in its entirety.

FIELD OF THE INVENTION

This invention relates broadly to the metallurgical arts and moreparticularly to a method for making aligned eutectic structures in theform of very thin films.

BACKGROUND OF THE INVENTION

Eutectics are phenomena of nature. A simple binary eutectic system istypified by the metallic alloys of lead and tin. Pure elemental tinexhibits an equilibrium freezing point of 232° C. and pure elementallead exhibits an equilibrium freezing point of 327° C. With but oneexception, alloys of tin and lead solidify and melt over a temperaturerange. The temperature at which a lead-tin alloy begins to solidify willbe less than the freezing temperature of elemental lead and may also beless than the freezing temperature of elemental tin.

The exception referred to above is an alloy of 38.1 wt. % lead and 61.9wt. % tin. This alloy is the alloy of the eutectic composition. Theeutectic alloy will freeze, under equilibrium conditions, at theeutectic temperature of 183° C. Also, under equilibrium conditions, thesolidification of lead-tin alloys of non-eutectic composition will becompleted at the eutectic temperature.

Eutectics exhibit a variety of structures. Such terms as lamellar,plate-like, rod-like, discontinuous, and divorced are commonly used todescribe the physical appearance of eutectics. The eutectic structure isdependent upon many factors including the components of the alloysystem, the nature and quantity of any impurities present, and the rateat which they are formed. For example, the eutectic of an alloy systemmay exhibit a regular periodic array of lamellae, or plates, whensolidified at near equilibrium rates. As the solidification rateincreases, the width of the lamellae will generally decrease, theperiodicity will generally become more irregular and individual lamellamay terminate abruptly or branch into one or more lamella thus creatingfaults in the otherwise periodic uniform structure. At very rapid ratesof solidification, the near-equilibrium lamellar structure may breakdown completely and form a new structure with a markedly differentappearance.

Eutectics are found in metallic, ceramic, and organic systems and neednot be formed from elements, i.e., combinations of compounds may formeutectics. Transparent organic eutectics having lamellar microstructuresand the method of making them are described, for example, in U.S. Pat.No. 3,484,153 to Hunt and Jackson. A binary eutectic formed from twoelements, or compounds, is the simplest eutectic and more complexeutectics, e.g. ternary and quaternary, are also observed.

Eutectics have many unique properties which make them candidates formany structural and non-structural applications. An example of the useof eutectics in a structural context is the report by Bruch et al. inthe Proceedings of the Conference on In Situ Composites-III (Ginn CustomPublishing, p. 258, 1979), that the eutectic alloy NiTaC-13 has beendirectionally solidified in the form of jet engine turbine blades andsuccessfully engine tested. The same Proceedings contain several paperson eutectics for nonstructural applications. The first paper in theseries on nonstructural applications is the one at p. 171 by Yue whichreviews the use of directionally solidified eutectics for electronic,magnetic, thermomagnetic, and superconducting applications.

Eutectics may be produced in bulk, as exemplified by the turbine bladediscussed above, or in thinner sections for nonstructural applicationsor for academic purposes such as the study of solidification mechanics.Eutectics in bulk form have been grown under unidirectional coolingconditions by such means as the Bridgman, Czochralski, zone levelling orfloating zone techniques.

Various techniques have been used previously to produce eutectics inthinner sections. Generally, the prior art techniques produced filmsthat were poor in quality, e.g., were non-uniform in thickness, were notfault-free over large regions, and exhibited poor alignment of thelamellae relative to the lateral surfaces.

Albers and Van Hoof report, for example, (Journal of Crystal Growth, 18,p. 147, 1973) use of a modified Czochralski technique to produce filmsof the Cd-Zn eutectic. Those films were extracted from the melt byimmersing a form, such as a wire loop, or a substrate into the melt andslowly withdrawing the form or substrate from the melt. The films ofAlbers and Van Hoof were not fault-free, appeared non-uniform inthickness, and exhibited an anomalous relationship between interlamellarspacing and pulling rate which is possibly indicative of non-uniformheat flow during the extraction process. Takahashi and Ashinuma (Jnl. ofInst. for Metals, 87, p. 19, 1958-59) used a technique similar to thatof Albers and Van Hoof to produce thin films of the Pb-Sn eutectic. ThePb-Sn films produced by Takahashi and Ashinuma were irregular inthickness and were not fault-free over large areas, but were suitablefor their purposes which was the study of the eutectic by means of theelectron microscope.

Another technique for producing thin eutectic structures is that ofDhindaw et al., reported at page 60 of the above-referenced Proceedings,wherein lead-cadmium and lead-tin eutectic alloys were encapsulated instainless steel or quartz capillaries. Dhindaw et al. report, interalia, that as the distance between the walls of the capillary decreased,there was an increased tendency for the lamellae of the eutecticsinvestigated to align perpendicular to the walls at the walls and toform parallel plates aligned perpendicular to the walls in the regionbetween the walls. That behavior was attributed to a constraining effectat the walls. As the distance between the walls increased, theconstraining effect at the walls reportedly became less effective inmaintaining the perpendicular alignment of the lamellae at the wallsresulting in the observed increased non-perpendicularity at the walls.That effect, in turn, caused the lamellae in the region between thewalls to form at increased angles to the walls and to exhibit anincreased tendency for fault formation.

BRIEF DESCRIPTION OF THE INVENTION

A method for the preparation of aligned eutectics as thin films isprovided. Briefly described the method includes the steps of depositingsequentially the components of the eutectic alloy system as overlyingthin planar layers on at least a portion of a substrate, which is inertrelative to the eutectic alloy system, to form a preform; creating amolten zone which extends through the planar layers of the eutecticcomponents and is contiguous with a portion of the top surface of thesubstrate; and moving the molten zone across at least a portion of thepreform causing the components to melt at the leading edge of the zone,mix in the molten zone and solidify as the thin film eutectic at thetrailing edge of the zone. Heat from a line heater or laser, preferablyfocused to form a line source of heat having a narrow width, is used toestablish the molten zone.

Using the method of the invention and a focused line heater as the heatsource, fault-free and cell-free lead-tin eutectic films as thin as 2microns with interlamellar spacings as small as 1.8 microns have beenproduced. Also, using the method of the invention, and a focused laseras the heat source, fault-free and cell-free lead-tin eutectic films asthin as 2 microns with an interlamellar spacing of 0.45 microns andfault-free and cell-free lead-cadmium eutectic films as thin as 2microns with interlamellar spacings as small as 0.1 micron have beenproduced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic perspective representation of a typical preform.

FIG. 2 is a schematic side view in cross section illustrating oneapparatus used for the production of thin film eutectics in accordancewith the present invention.

FIG. 2A is an enlarged perspective representation of the shutters usedto vary the spatial power distribution of the line source of heat shownin FIG. 2.

FIG. 3 is a schematic representation of the top surface of a preformpartly converted to a eutectic thin film in accordance with the methodof this invention.

FIG. 3A is a schematic cross section of the thin film eutectic of FIG. 3taken along line 3A--3A.

FIG. 4 is a scanning electron micrograph at 7000× of a 4-micron thickPb-Sn eutectic film solidified at 2×10⁻³ cm/sec. The interlamellarspacing is 1.8 microns, the lighter Pb-rich phase is 0.72 microns wide,and the darker Sn-rich phase is 1.08 microns wide.

FIG. 4A is a schematic view in cross section of the thin film eutecticof FIG. 4 taken along line 4A--4A.

FIG. 5 shows the spatial temperature distribution beneath an unmodifiedline heater at a power level of about 20 watts/cm with the graphicalcalculation of the theoretical thermal gradient imposed thereon. Alsoshown is the effect of increased power level on the shape of the spatialtemperature distribution.

FIG. 5B shows the spatial temperature distribution beneath a line heatermodified by shutters having a 3 mm gap between the shutters with thegraphical calculation of the theoretical thermal gradient imposedthereon.

FIG. 6 is a schematic perspective drawing of the generation of a linesource of heat using a rotating polygonal mirror and a laser beam.

FIG. 7 is a graph of film thickness versus solidification rate showingthe conditions under which a lamellar structure will be formed.

FIG. 8 is a graph of interlamellar spacing versus solidification ratefor several lead-tin and lead-cadmium thin film eutectics made witheither a quartz lamp or laser beam heat source in accordance with themethod of this invention.

FIG. 9 is a photomicrograph at 1000× of a 2 micron thick Pb-Sn eutecticfilm solidified at 5×10⁻² cm/sec showing the formation of cellboundaries.

DETAILED DESCRIPTION OF THE INVENTION AND THE PREFERRED EMBODIMENTS

With reference to the Figures, beginning with FIG. 1, the invention maybe readily explained. Assuming selection of the system whose eutectic isto be prepared as a thin film by the method of this invention, the firststep is the selection and preparation, if necessary, of substrate 1. Thechief factors to be considered in the selection of a suitable substrate1 are thermal conductivity and chemical inertness with the eutecticsystem selected. Suitable substrate materials are, for example, Pyrex®glass microscope slides, alumina, mica, and silicon. The effects ofthermal conductivity are discussed below.

The geometry of the substrate will primarily be determined by theultimate use of the thin film eutectic solidified thereon, but will atleast be characterized by having top 2 and bottom 3 major opposedsubstantially parallel surfaces and an outer peripheral edge area 4interconnecting the major surfaces. Smoothness of substrate surface 2,which will contact the eutectic thin film, is a factor as asperities inthe surface 2 will disrupt the solidification process and produce faultsor other defects. Preferably, surface 2 should be smooth to within atleast about one-tenth of the film thickness. Also, based on heat flowconsiderations, the surface 3 opposite surface 2 should be flat andsubstantially parallel to surface 2.

Next, the thicknesses of the materials, i.e., components, of theeutectic system are calculated, per unit area of substrate, as a ratioaccording to the following formula:

    t.sub.1 ρ.sub.1 W.sub.1 =t.sub.2 ρ.sub.2 W.sub.2 = . . . =t.sub.n ρ.sub.n W.sub.n                                       (1)

where

W₁ =weight percent of component 1 in the eutectic

W_(n) =weight percent of the n^(th) component in the eutectic

ρ₁ =density of component 1

ρ_(n) =density of the n^(th) component

t₁ =thickness of the layer of component 1

t_(n) =thickness of the layer of the n^(th) component

and converted to actual thicknesses by use of the formula:

    t.sub.film =t.sub.total =t.sub.1 +t.sub.2 + . . . +t.sub.n (2)

For a binary system, equation (1) reduces to ##EQU1## The calculationalmethod shown above is more accurate than calculations from the phasediagram based on the so-called lever rule and is, therefore, preferred.

The starting materials should be as pure as possible, preferably "4-9s"or purer, as impurities tend to disrupt the heat and mass balances ofthe solidifying eutectic thereby forming defects, e.g., faults.Contamination of the substrate and the materials of the eutectic is tobe avoided. The practice of clean room conditions, such as are known tothose skilled in the art of the manufacture of semiconductor devices, ispreferable.

The eutectic materials and the substrate are transferred to suitableapparatus (not shown) for the evaporation and deposition of thematerials onto substrate 1. In a vacuum, preferably less than or equalto 10⁻⁶ torr, the materials of the eutectic are evaporated and depositedsequentially in overlying planar layer-like fashion onto the substrate.In FIG. 1 there is shown schematically a first component layer 5 of abinary eutectic system deposited upon substrate 1 and the secondcomponent layer 6 deposited upon layer 5. Preferably, the thickness ofeach layer is within ±10% of that calculated with Equations (1) and (b2), although for some eutectic systems the acceptable tolerance may beless than ±10%.

Optionally, a cover layer 7, as shown in FIG. 1, may be provided on topof the deposited eutectic components. The material property requirementsof cover layer 7 are the same as those of substrate 1. The cover layer 7may be provided by depositing a refractory metal oxide subsequent to thedeposition of the layers of the eutectic material or may be anotherpiece of the same material as substrate 1 laid upon the depositedcomposite. The cover layer 7 is desirable to prevent oxidation of theeutectic during subsequent processing. Oxidation may also be avoided bypracticing the invention in a vacuum or inert atmosphere. With orwithout cover layer 7, a completed preform 10 has been fabricated atthis stage.

The prepared eutectic preform 10 is next placed in an apparatus, such asthat shown schematically in FIG. 2, for the next steps in thepreparation of the eutectic thin film. The apparatus of FIG. 2 consistsof at least a heat source 20 capable of projecting a beam of heat 25onto preform 10, a heat sink 30 and means 40 for smoothly translatingthe heat sink 30 and preform 10 mounted thereon beneath heat source 20at a determinable uniform rate. Sufficient heat is applied from heatsource 20 to form a narrow zone 50 of width W and longitudinallyextending length L, as shown in FIG. 2 and in FIGS. 3 and 3A in moredetail. FIG. 3 is, in part, a schematic representation of the topsurface of a preform partly converted to a eutectic thin film inaccordance with the method of this invention.

As shown in FIG. 3A, molten zone 50 will be coextensive with at least aportion of the substrate 1 and cover layer 7, if cover layer 7 ispresent. If cover layer 7 is not present, molten zone 50 will rest on atleast a portion of substrate 1 and will be otherwise bounded by theunmelted eutectic material layers 5 and 6 of preform 10. By operatingmeans 40, preform 10 is transversed beneath the stationary heat sourcethereby, in effect, moving molten zone 50 across preform 10. As moltenzone 50 traverses preform 10, the components 5 and 6 of the eutectic aremelted at the leading edge 52 of zone 50, mixed together in zone 50, andsolidified at the trailing edge 53 of zone 50 in the desired eutecticpattern 54. After the desired amount of material has been melted andsolidified, the traversing motion of means 40 is stopped and heat source20 is turned off whereupon molten zone 50 freezes in situ. In FIGS. 3and 3A, molten zone 50 is shown after traversing a distance Z, in thedirection of the arrow, from starting location 51.

FIG. 4 is an electron photomicrograph at 7000× of a typical two phasethin film lamellar eutectic made by the method of this invention, e.g.,schematic region 54 of FIG. 3 rotated through an angle of 90°. Theinterlamellar spacing, λ, defined as the distance between the center ofone lamella to the center of the nearest adjacent lamella of the sametype, is shown in FIG. 4. Those skilled in the metallurgical arts willunderstand that the compositions of the lamellae, denoted as 55 and 56in FIG. 4, will be at or below the terminal solid solutions of therespective phases of the alloy system.

The spatial arrangement of the lamellae of the eutectic film of FIG. 4is shown in more detail in FIG. 4A which is a cross-sectional schematicview taken along line 4A--4A of FIG. 4. On FIG. 4A there is shown theinterlamellar spacing, the widths λ_(A) and λ_(B) of lamella 55 and 56,respectively, and the resultant film thickness, t_(film).

Generally, the thickness of the resultant film is equal to the sum ofthe thicknesses of the component layers. A desirable feature of themethod of this invention, compared to such prior art methods as meltextraction, is that large areas of thin film, e.g., 2 inches square, butwithout practical limitation thereon, having a uniform thicknessthroughout can be reproducibly produced. Additionally, as will bedescribed below in more detail, the interlamellar spacing and filmthickness can be independently selected.

The lamellae extend between and terminate in the substantially paralleland generally planar top 57 and bottom 58 surfaces of the thin film. Ageneral and desirable characteristic of the lamellar eutectics producedby the method of this invention is that the center lines of thelamellae, when viewed in cross section, are substantially parallel tothe thickness dimension, i.e., the lamellae intersect the top 57 andbottom 58 surfaces of the thin film at substantially right angles. Thisdesirable features makes the thin film eutectics of this inventionuseful as diffraction gratings in general and, as disclosed in theabove-referenced Ser. No. 253,985 application, particularly useful asmasks for X-ray lithography.

As used herein, the term "fault-free" means an absence of lamellaeterminations, or branches, over the primary region of interest whichencompasses a region at least 100 lamellae square. Also, the term"cell-free" implies a structure produced by a planar solid-liquidinterface resulting in substantially parallel alignment of the lamellaeof a lamellar eutectic over the entire area of the solidified thin filmwhich typically measured, but is not limited to, an area 2 inchessquare.

The interlamellar spacing is a function of the growth, i.e.,solidification, rate and generally follows the empirical relationship

    λ.sup.2 V=constant                                  (4)

where V equals the growth rate. Generally, the growth rate is equal tothe rate at which molten zone 50 is traversed across preform 10. In thecase of the Pb-Sn eutectic system, the constant has been found [Clineand Livingston, Trans. TMS-AIME, 245, 1987 (1969)] to be equal to about3.8×10⁻¹¹ cm³ /sec for solidification in bulk, i.e., thick, sections.The thin films of this invention generally followed the λ² Vrelationship. The mean value of the constant (5.4×10⁻¹¹ cm³ /sec) forthe Pb-Sn thin films produced by the method of this invention wascomparable to that for the bulk, however, there was considerable scatterabout the mean which is indicative of an increased sensitivity of thethin film eutectics to local thermal conditions compared to eutecticssolidified in bulk.

Several factors govern the selection of substrate 1 and heat sink 30.The thermal conductivity of heat sink 30 must be greater than that ofsubstrate 1. Enough heat must be supplied to melt the eutecticcomponents deposited on substrate 1. However, if too much heat issupplied, or not dissipated rapidly enough, the melted eutectic willtend to coagulate and form small droplets of the liquid metal. Routineexperimentation may be required to strike the proper balance between theheat supplied and removed in relation to the materials of the eutecticsystem, substrate 1 and heat sink 30.

Substrate 1 must make good uniform contact with heat sink 30 or therewill be non-uniformity of the heat flow pattern and a non-uniform, e.g.,faulted, thin film eutectic may result. To promote heat transfer andestablishment of a steep thermal gradient, G_(L), in molten zone 50,heat sink 30 is preferably equipped with means, such as internalpassageways, for circulation of a suitable cooling fluid. Copper is ahighly desirable material for heat sink 30 and its effectiveness may beenhanced by proper preparation of the surface closest to the preform 10such as by dressing with a fly cutter. Overall uniformity of heattransfer may also be enhanced by placing a flat quartz plate 35 betweenheat sink 30 and preform 10.

The thermal gradient in the liquid, G_(L), and the growth, orsolidification rate R, interact to determine the quality of eutecticformed. For example, for a lamellar eutectic, increased values of G_(L)/R will ensure that a cell-free lamellar, i.e., parallel plate,alignment is maintained over extensive distances. Increased values ofG_(L) permit increased values of R which, in turn, yield decreases inthe interlamellar spacing. The thermal gradient G_(L) is primarily afunction of the spatial distribution of heat from source 20 and thethermal characteristics of the system consisting of the materials of theeutectic, substrate 1 and heat sink 30.

A line heater, such as that shown in FIG. 2, has been found to be aneffective heat source 20 with suitable modifications including a linevoltage regulator to minimize power fluctuations. One such line heateris that manufactured by Research, Inc. of Minneapolis, Minnesota(Catalogue #5215-10). The line heater of FIG. 2 consists primarily of alamp 21 situate at the focal point of an aluminum elliptical reflector22 which has cooling channels 23 therein. It has been found that aquartz lamp is an effective heat source 21.

It was determined during the course of the making of this invention thatthe unmodified commercial line heater produced a lower than desirablethermal gradient within molten zone 50. FIG. 5A shows (curve A) thethermal profile beneath an unmodified line heater 20 equipped with aquartz lamp. The temperature profile of FIG. 5A was measured with a 0.01cm. diameter thermocouple cemented to a Pyrex® slide to simulate thethermal environment of the film. The typical thermal profile, i.e.,spatial power distribution, of FIG. 5A, produced a simulated thermalgradient of about 200° C./cm. at the melting point (183° C.) of thelead-tin eutectic at an applied power of approximately 20 watts/cm. Thedistribution of FIG. 5A actually resulted in a 0.3 cm. wide melt zone 50in a 4 micron thick lead-tin thin film eutectic at a solidification(growth) rate of 4×10⁻³ cm/sec. Changes in power level will affect thegradient. Increased power, for example, will raise the peak temperatureand broaden the distribution, as shown by the dashed lines (curve B) onFIG. 5A; the net effect of which will be to decrease the gradient. Forthin lead-tin eutectic films, e.g., those less than or equal to about 8microns in thickness, the temperature distribution was found to beprimarily a function of the type and thickness of the substrate materialand independent of the eutectic film thickness.

The thermal gradient was improved during the course of the making ofthis invention by placing adjustable shutters 24 between line heater 20and preform 10 as shown in FIGS. 2 and 2A. The best material forshutters 24 was found to be highly polished aluminum. The gap, 2K,between the shutters is primarily a function of the thickness, t_(s),and material of substrate 1 and the power, i.e., heat intensity, of lamp21 and is readily determined by trial and error. The power and gap areadjusted to provide sufficient heat to melt the eutectic components withthe narrowest thermal profile.

In FIG. 5B there is shown the simulated thermal gradient (525° C./cm)made possible by use of shutters 24 having a gap, 2K or 3 mm. at a powerlevel of 500 watts. The simulated thermal profile of FIG. 5B was made inthe same manner as that of curve A of FIG. 5A. The simulated thermalprofile of FIG. 5B actually resulted in a 0.2 cm. wide molten zone 50 ina 2 micron thick lead-tin thin film eutectic at a solidification(growth) rate of 4×10⁻³ cm/sec. Increased temperatures may be producedby the use of heat sources such as arc lamps which provide more heatthan those of the quartz lamp.

A laser is also a suitable, although more expensive, heat source 20 thanthe line heater. The narrower molten zone 50 created by a laser makespossible higher thermal gradients in the molten zone 50 and,consequently, eutectics with thinner lamellae and narrower interlamellarspacings. Replacement of the line heater with a laser as heat source 20requires means for spreading the beam into a line heat source. The useof optics to slowly scan or raster the laser beam across preform 10 hasbeen found to result in objectionable surface perturbations. A simplenon-mechanical solution is to provide a cylindrical lens in the path ofthe laser beam between the laser as heat source 20 and the preform 10 toconvert the circular beam into a thin line source of heat. It has beenfound, however, that the beam intensity was not uniform along the lengthof the line when the lens system was used, i.e., there was a decrease inpower at the ends compared to the center of the line of heat.

The laser beam 60 may be scanned rapidly enough through the use of alens-mirror system, such as the rotating polygonal mirror 61 shown inFIG. 6, to create a line source of heat. A further advantage of the useof a laser as heat source 20 is that preform 10 may be kept stationaryand molten zone 50 traversed across preform 10 by the use of additionaloptical scanning means. By keeping the preform 10 stationary, it ispossible to minimize disruption of the structure by external mechanicalvibrations. The use of the rotating mirror, however, adds to the overallcost of the system and requires that the optical system be kept inperfect alignment.

EXAMPLE I

A systematic investigation of the stability of the lamellar structure atdifferent film thicknesses and solidification rates was conducted andthe results are shown in FIG. 7. Thin films of Pb and Sn weresequentially deposited on Pyrex® microscope slides in an electron beamevaporator. Evaporation was performed in a vacuum of 10⁻⁶ torr fromsources of 99.999 percent pure material. First the Pb film was depositedon the freshly cleaned glass slide and then, without breaking vacuum,the Sn film was deposited on top of the Pb layer. The total filmthickness was varied between one and 8 microns to study the effect offilm thickness at the eutectic composition. The film thicknesses werecalculated in accordance with equations (1) and (2), and, in all cases,the Pb film was 28% of the total film thickness. A second Pyrex®microscope slide (similar to the substrate) was used to cover the filmand protect it from oxidation during the solidification process.

The samples were individually directionally solidified at the ratesshown on FIG. 7 utilizing a line heater as shown in FIG. 2. Aquartz-iodine lamp located at one focus of an elliptical reflectorproduced a line source of heat about 0.3 cm. wide, and longitudinallyextending about 5 cm, at the surface of the film after passing through ashuttered gap 0.3 cm. wide. Each glass slide was individually placed ona quartz plate that rested on a flat water-cooled copper heat sink.Thermal contact between the quartz plate and the copper heat sink wasincreased a factor of two by machining the surface of the copper flatwith a fly cutter.

After solidification, the covering glass slide was readily removed as itdid not stick to the thin film eutectic. The interlamellar spacings, λ,and widths of the individual lamella were measured by means of opticaland scanning electron microscopy. Since the tin- and lead-rich phases,i.e., the alternate lamellae, have a different reflectivity, the phasesmay be distinguished without any metallographic preparation of thesurface. Contrast was improved using polarized light, since tin isoptically active. A scanning electron microscope was used to reveal thestructure with higher resolution than was possible with lightmicroscopy. A typical scanning electron micrograph is that of FIG. 4 at7000× wherein the darker tin-rich phase is 1.08 microns wide, thelighter lead-rich phase is 0.72 microns wide and the interlamellarspacing is about 1.8 microns. The structure of FIG. 4 is alsorepresented by data point I on FIG. 8 which is a graph of interlamellarspacing versus solidification rate. In the scanning electron microscopethe lead-rich phase appears lighter than the tin-rich phase which wasopposite to that of light microscopy, where the lead-rich phase wasdarker.

The structure of the Pb-Sn eutectic film was a function of the filmthickness and growth rate as shown in FIG. 7. Above a criticalcombination of film thickness and growth rate, curve C of FIG. 7, thestructure was lamellar and below the critical combination the structurewas observed to be irregular.

EXAMPLE II

Using the same general procedure described in Example I, a thin film 2microns thick of lead-tin eutectic was made. The solidification ratewas, however, increased to 5×10⁻² cm/sec. The structure formed is shownin FIG. 9 and the specimen is identified as data point II on FIG. 8. Thecurvature of the lamellae observable in FIG. 9 is due to a departurefrom a planar liquid-solid solidification interface due to the increasein solidification rate over that of Example I. In areas away from thecurved plates at the cell walls the lamellae are parallel and have aninterlamellar spacing of about 1.05 microns.

EXAMPLE III

Using the same general procedure as described in Example I, a 2 micronthick lead-tin eutectic was produced by the method of this inventionusing a laser heat source. The laser employed was a Quantronix Model 117Nd:YAG 90 watt (maximum) continuous power laser. The circular laser beamwas converted and focused into a line heat source by placing acylindrical lens with a 50 mm. focal length in the path of the beam. A200 micron wide molten zone about 4 mm in length was established andmoved with a velocity of 5.4×10⁻³ cm/sec. A fault-free and cell-freeeutectic with an interlamellar spacing of 1.0 micron was formed. Thisexample is indicated as point III on FIG. 8. The structure of thiseutectic did not exhibit the curvature and cell walls exhibited by theeutectic of Example II. The lack of curvature is attributed to thehigher thermal gradient, G_(L), in the molten zone, and concomitantplanar solidification interface, which resulted from the narrower linesource of heat produced by the laser compared to that produced by theline heater of Example II.

EXAMPLE IV

Using the same method described in Example III another lead-tin eutecticwas solidified at a rate of 2×10⁻² cm/sec. The resulting cell-freestructure had interlamellar spacings between 0.45 and 0.65 micronsdepending upon the area of the film examined. This variation ininterlamellar spacing was due to local variations of the laser power.The data points of this example are identified as points IV and V onFIG. 8. Again, the cell-free structure was due to the higher thermalgradient that resulted from the use of the laser as the heat sourcecompared to the thermal gradient produced by the line heater.

EXAMPLE V

Using the same method as Example IV a lead-cadmium thin film eutectic 2microns thick was solidified at a rate of 1.45×10⁻¹ cm/sec. Theinterlamellar spacing was near the limit of resolution of the scanningelectron microscope but was measured to be 0.1 micron. The width of thecadmium-rich phase was below the resolution limit of the scanningelectron microscope, but was calculated at 0.025 microns from the phasediagram. The lead-cadmium eutectic system is expected to be moreamenable to processing as a thin film by the method of this inventionsince, in contrast to the lead-tin system, there is a preferredcrystallographic relationship between the lamellar phases. The width ofthe lead phase is about 2/3 of the width of the tin phase in thelead-tin eutectic and about three times the width of the cadmium phasein the lead-cadmium eutectic. The Pb-Cd eutectic of this Example isdenoted by data point VI on FIG. 8.

While the invention has been particularly shown and described withreference to several preferred embodiments thereof, it will beunderstood by those skilled in the art that various changes in form anddetail may be made therein without departing from the true spirit andscope of the invention as defined by the appended claims.

I claim:
 1. A method for making eutectics as thin films comprising thesteps of:(a) depositing sequentially the components of the eutecticsystem at a purity level of at least 99.99% as overlying thin planarlayers on at least a portion of a substrate, said substrate having topand bottom major opposed substantially parallel surfaces and an outerperipheral edge area interconnecting said major surfaces and being of amaterial which is substantially inert relative to said eutectic system,forming thereby a preform; (b) creating a longitudinally extendingmolten zone of said components by impinging on said preform a beam ofheat from a quartz lamp heat source within a line heater, said moltenzone extending through said planar layers and being contiguous with atleast a portion of said top major surface of said substrate; and (c)moving said molten zone across at least a portion of said preformmelting thereby the components of said eutectic alloy at the leadingedge of said zone, mixing said components in said zone, and freezingsaid components at the trailing edge of said zone, forming thereby aeutectic thin film of the eutectic composition of said components. 2.The method of claim 1 wherein the material of said substrate is oneselected from the group consisting of glass, mica, silicon, and alumina.3. The method of claim 1 wherein said top surface of said substrate issmooth to within at least about one-tenth of the thickness of the sum ofthe thicknesses of said planar layers.
 4. The method of claim 1 whereinsaid molten zone is moved across said preform by means of an opticalsystem.